1. Field of the Invention
The present invention relates to a fibrous solid electrolyte for use in solid electrochemical elements including a solid cell, a shaped product thereof and a process for forming the fibrous solid electrolyte, and more particularly to a fibrous lithium ion conductive solid electrolyte having the best ion conductivity, when its ion conductive species is lithium ions.
2. Related Art of the Invention
With recent development of portable appliances such as personal computers, portable telephones, etc., a demand for cells as their power source has been considerably increased. Particularly, lithium cells have been extensively studied in various fields as cells capable of giving a high energy density, because lithium is a substance having a small atomic weight and a large ionization energy.
On the other hand, the cells so far used in these fields are based substantially on a liquid electrolyte and thus it has been impossible to eliminate such problems as leakage of liquid electrolyte, etc. To solve these problems, thereby enhancing the reliability and also to make the elements smaller and thinner, attempts for replacing the liquid electrolyte with a solid electrolyte and making an entirely solid cell have been extensively made in various fields.
The above-mentioned lithium, when brought into an abnormal state, has a fear of ignition due to the high energy density. Thus, development of an entirely solid lithium cell using a solid electrolyte made from a non-combustible solid has been desired to ensure the safety of the cell. Lithium halide, lithium nitride, oxy acid salts of lithium, and their derivatives are known as solid electrolytes for use in such a cell. Furthermore, researches on sulfide glassy solid electrolytes such as Li.sub.2 S-SiS.sub.2, Li.sub.2 S-P.sub.2 S.sub.5, Li.sub.2 S-S.sub.2 S.sub.3, etc. and lithium ion conductive solid electrolytes prepared by doping the above-mentioned glassy solid electrolytes with a lithium halide such as LiI, etc., or with an oxy acid salt of lithium such as Li.sub.3 P.sub.4, etc., particularly on their physical properties have been made in the world owing to their high ion conductivity such as 10.sup.-4 to 10.sup.-3 S/cm.
However, these solid electrolytes are inorganic solid powders and thus pelletization by press molding, etc. are indispensable for their application to cells. The resulting pellets are too hard and fragile to make electrolytes having a smaller thickness and a larger area. Furthermore, the resulting electrode is fragile when assembled with the positive pole and the negative pole.
To solve this problem, it has been proposed to integrate the inorganic solid electrolyte powders with an elastic polymer, thereby increasing the processability of the solid electrolyte and produce a shaped solid electrolyte product having a desired shape such as a sheet form from the integrated powders.
According to one of the proposed processes for producing the shaped solid electrolyte product, the surfaces of solid electrolyte powders are coated by an elastic polymer dissolved in an organic solvent and then the resulting mixture of particles of the solid electrolyte powders are bonded to one particle to another by press molding, followed by removal of the organic solvent from the shaped product by evaporation. However, in case of a bulk electroconductivity-dominating solid electrolyte such as a glassy solid electrolyte, the ion conductivity will be lowered to the extreme, and thus it has been difficult to use this process.